The DOBA is a high acid crude oil originating from the Chad region of West Africa. The DOBA is known to contain calcium naphthenate and the amount of calcium naphthenate varies over a range from about 150 to about 700 ppm. In a typically supplied DOBA crude oil, the amount of calcium naphthenate may vary from about 250-about 300 ppm.
The DOBA is a heavy high acid crude oil with Total Acid Number [TAN] ranging upwards of 4.0 mg KOH/gm of sample and the API gravity is about 19. The sulfur content in DOBA is very low to nil.
The DOBA is typically a crude oil with a lot of residue in it and for proper blending, typically internationally, refiners blend it with very light crude oil or condensates to increase the API of the resulted blend to more than 30. Such blending with light crude oil or condensates helps to create sufficient light ends to help achieve product yields for the crude distillation unit. The most of light crude oil or condensates thus selected generally have very little to nil sulfur contents, which means the overall sulfur content still remains very low. Further, the H2S being oil soluble is not present in relatively higher quantities in these types of blends.
The inventor of present invention has observed that if solution of calcium naphthenate in an organic solvent, for example toluene having concentration of Ca of about 2247 ppm is treated with equal weight of water by heating to about 130 degree C., that is, being equal to a crude oil having very little to nil sulfur contents or say in absence of sulfur compound in a Parr autoclave under autogenous pressure, and separated into organic and aqueous layers in a separating funnel, no black layer is formed at the interface in presence of water or say in presence of very little to nil sulfur contents, being less than 0.2%. When organic layer, as separated, was dried by evaporating toluene, its acid value was found to be very low—about 48.36 (mg KOH per gm). The low Acid Value indicates that calcium naphthenate does not hydrolyze appreciably merely in presence of water or say in presence of very little to nil sulfur contents, being less than 0.2%.
The inventor of present invention has observed that when DOBA or its blend containing calcium naphthenate with no sulfur or sulfur contents being very little [less than 0.2%] is treated with additives as known in the art, for example with glycolic acid it does not hamper removal of metals including calcium from such DOBA crude oil or its blend.
However, the inventor has experimentally found that even with very little to nil sulfur contents, the efficiency of glycolic acid to remove calcium from crude oils containing calcium naphthenate is very low, which surprisingly reduces further if the pH of the crude oil increases to about 6 or to about 11 due to presence of ammonium hydroxide or ammonia or other nitrogen compounds in wash water for desalter.
The inventor of present invention has also observed that when DOBA or its blend containing calcium naphthenate with no sulfur or sulfur contents being very little [less than 0.2%] is treated with additives as known in the art, for example with malic acid, it does not hamper removal of metals including calcium from such DOBA crude oil or its blend.
However, the inventor has experimentally also found that even with very little to nil sulfur contents, the efficiency of malic acid to remove calcium from crude oils containing calcium naphthenate is very low, which surprisingly reduces further if the pH of the crude oil increases to about 6 or to about 9 or to about 11 due to presence of ammonium hydroxide or ammonia or other nitrogen compounds in wash water for desalter.
Inventor's above findings confirm that prior art additives—glycolic acid and malic acid are effective to remove calcium from DOBA crude oil, but with very low efficiency, and this efficiency, surprisingly, reduces further if pH of the crude oil or wash water increase to about 6 or to about 9 or to about 11.
The inventor has also found that efficiency of prior art additives—maleic anhydride, citric acid, D-gluconic acid to remove calcium from DOBA crude oil is very low particularly when pH of the crude oil or wash water increase to about 6 or preferably to about 9 or to about 11.
Accordingly, the industry processing DOBA or its blends containing calcium naphthenate even with very little to nil sulfur contents faces serious problems in removing calcium from such oil or its blends at pH varying from about 6 to about 11 of the wash water for desalter.